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UCC25640EVM-020: Power management forum

Rahamadullah A
Prodigy 160 points
Part Number: UCC25640EVM-020
Other Parts Discussed in Thread: UCC256404

Tool/software:

Hi TI team

Greetings!

We are planning to use 400V to 12V LLC converter using UCC256404 IC. 

In order to have tight  CTR tolerance wrt ageing we have planned to use optoemulator ISOM8110DFGR.

In this optoemulator if i allow If = 5mA then my CTR variation is narrowed to only between 100% to 155%.

But in that case UCC256404 FB pin could source only maximum of 85uA. In that case how do i use the optoemulator ?

If forward current of 5mA is not possible then for atleast 2mA of forward current how do i design my FB pin of UCC256404.

Also if i bias FB pin externally using RVCC will it disturb the feedback information of UCC256404 ( iam sourcing externally instead of pulling current from FB pin)

Please advice in this regard.

Regards

Rahamadullah

  • 0
    TI__Guru* 86286 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86286 points

    Hello,

    I think the CTR is based on 0.5 mA and not 5 mA for ISOM8110DFGR .  However, 500 uA is still above the 82 uA max control current.

    The UCC25640x devices do use an extremely low current (IFB) to control the LLC oscillator frequency.  0 to 82 uA is used to control the min to maximum frequency range.  This low bias FB current scheme was developed to reduce standby power by reducing opto current.  The only problem with this is the 0 to 82 uA used to control the frequency is below most opto isolators characterized CTR ranges.  However, most opto isolators can still work with this low current.  It just requires testing and characterizing the opto isolators CTR that is being used in your design.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86286 points

    Hello,

    I discussed this issues with my colleagues today and they had mention opto isolator VO618A-3 is characterized down 100 uA  diode forward current with a CTR of 0.1.  This would be a collector current of 10 uA.  The following link will bring you to the data sheet for this opto oscillator.

    https://www.vishay.com/docs/83432/vo618a.pdf

    Regards,